1. Field of the Invention
The invention relates to a treatment process that forms a sparingly soluble chromate coating on steel, especially steel sheet, coated with zinciferous metal. More specifically, the invention relates to a treatment process that produces a sparingly soluble chromate coating or film which exhibits an excellent corrosion resistance, alkali resistance, water resistance, paintability (paint film adherence and post-paint corrosion resistance), and in particular an excellent coatability with water-based paints. The treatment process of the invention forms these films on the surface of, for example, electrogalvanized steel, steel electroplated with zinc alloy, hot-dip galvanized steel, and steel hot-dip coated with zinc alloy (hereinafter collectively referred to as zinciferous metal-plated steel).
2. Statement of Related Art
Older chromate treatment technologies used aqueous solutions of chromic acid or dichromic acid in order to improve the corrosion resistance and paintability of zinciferous metal-plated steel sheet. Various types of chromate treatment processes and chromate film-forming processes have been proposed more recently in order to obtain ever greater levels of corrosion resistance and paintability. The essential features, advantages, and disadvantages of the various elements of this prior art are considered below.
Japanese Patent Publication Number Sho 52-2851 [2,851/1977] discloses a process for improving rust inhibition through the formation of a chromate film on the surface of zinciferous metal-plated steel sheet. This particular process is a film-forming process that uses an aqueous solution of chromic acid, silica sol, and a small quantity of trivalent chromium ions. However, since hexavalent chromium is the main component in the chromate film laid down by this process, the film is hygroscopic and exhibits an inadequate rust-inhibiting activity.
The process taught in Japanese Patent Publication Number Sho 61-58552 [58, 552/1986] uses a chromate bath of silica sol-chromic acid-reduced chromic acid. However, when a surface-treated steel sheet carrying chromate film formed by this process is subjected to additional processing and then painted, the hexavalent chromium ions are easily eluted from the chromate film during the alkaline cleaning step that precedes painting. This results in a reduced corrosion resistance by the film.
The processes taught in Japanese Patent Application Laid Open Numbers Sho 58-22383 [22,383/1983] and Sho 62-83478 [83,478/1987] use silane coupling agent to chemically reduce hexavalent chromium ions in the chromate treatment bath. The films produced by these processes exhibit an excellent adherence to paint films. However, the chromate film generated by the former process exhibits a poor alkali resistance because the chromate treatment bath used in this process lacks phosphoric acid. The chromate film generated by the latter process suffers from an inadequate alkali resistance for the same reason.
Japanese Patent Application Laid Open Number Sho 63-96275 [96,275/1988] discloses a treatment process that uses a chromate treatment bath containing a specific type of organic resin (the hydroxyl group has been introduced in a particular quantity into the resin molecule). However, because the chromate film laid down by this process contains carboxyl-bearing organic resin as produced by chromic acid oxidation, this film again has an unsatisfactory alkali resistance. Another drawback to this process is the very low stability of its treatment bath, which occurs because the reaction between chromic acid and the hydroxyl-bearing organic resin proceeds even in the solution.
Chromating technology must now also take into consideration the various measures that have come into play in the last few years accompanying the increasing levels of human consciousness with respect to global environmental protection. These measures differ from previous pollution control efforts in that the various regulations required for global environmental protection are now being implemented on a world-wide scale. For example, regulations are under consideration that would halt the production of chlorinated solvents or would lead to a comprehensive reduction in the discharge of carbon dioxide and volatile organic compounds. In order to achieve reductions in the use of organic solvents, industry is therefore switching from solvent-based paints to water-based paints and from chlorinated solvent degreasing to water-based degreasers.
The use of water-based degreasers and water-based paints imposes novel performance requirements on chromate film-forming treatments for zinciferous metal-plated steel that are not imposed by the older technologies of solvent-based painting and chlorinated solvent degreasing.
Resistance to dissolution in water-based paints is one example of a new performance property required of the films produced by the chromate treatment of zinciferous metal-plated steel. When the chromate film is highly soluble in water-based paint, components of the chromate film will dissolve into the water-based paint and the resin portion in the paint will gel. This impairs the appearance of the paint, and the deterioration in the resin reduces the performance of the paint film. Moreover, a high resistance to water-based alkaline degreasers is required, and thus alkali resistance also becomes a necessity.
When a poorly alkali-resistant chromate coating is produced, the chromium ions will elute into the degreaser, and the direct discharge of degreaser effluent will then create a new source of environmental pollution. This necessitates treatment to remove the chromium ions present in alkaline degreaser effluent.
However, a chromate film that solves all these problems has yet to appear.